1. Field of the Invention
The present invention relates to III-nitride light-emitting diode and their forming methods.
2. Description of the Prior Art
Solid-state light sources based on white light emitting diode (LED) technology have gained much attention because of their tremendous potential for energy-efficient general illumination applications. In white LEDs, the luminous efficacy and color rendering can be controlled by light mixing of polychromatic (e.g., red, yellow, green, blue) emitters. At present, the indium gallium nitride (InGaN) compound semiconductor alloys are considered as the most promising material system for white LEDs since the direct band gaps of InxGa1-xN (0≦x≦1) can be continuously tuned from the near-infrared (0.6 eV, InN) to near-UV (3.4 eV, GaN) region, including the entire visible spectrum. However, this potential is limited by the dramatic drop in the InGaN emission efficiency at longer wavelengths. To date, efficient InGaN LEDs are only available in the blue region. Thus, monolithic white LEDs are typically realized by the luminescence down-conversion technique using yellow phosphors, such as cerium-doped yttrium aluminum garnet. The efficiency and light quality of phosphor-converted devices, however, are still imperfect due to the Stokes shift loss and limited color rendering. Furthermore, optimized color displays would require full-visible-spectrum emitters. Thus, a major current research focus is to improve the InGaN emission efficiency at longer wavelengths. Especially, the spectral range between 550 and 590 nm is the well known “green-yellow gap,” where the highest spectral response region of the human eye resides in and none of the existing semiconductors can be used to make high-efficiency LEDs.
The origin of the wavelength-dependent emission efficiency can be attributed mainly to the large lattice mismatch between InN and GaN (˜11%) and the polar nature of their crystal structure. High-quality InGaN LEDs based on conventional planar InGaN/GaN multiple quantum well structures are currently grown along the polar c-axis of the wurtzite crystal structure. Therefore, growth of high-In-content InGaN/GaN quantum wells would unavoidably result in a high density of defects and huge internal electrostatic (piezoelectric) fields (>1 MV/cm). The internal fields in the InGaN wells spatially separate the electron and hole wave functions, i.e., quantum confined Stark effect (QCSE), making highly efficient longer-wavelength LEDs difficult to achieve based on polar c-plane structures. For the blue (low-In-content) InGaN LEDs, the carrier localization phenomenon and ultrathin quantum well structures (about 2-4 nm in width for nearly all commercial InGaN LEDs or laser diodes) could alleviate the effects of high defect density and QCSE. Unfortunately, these are not applicable in the case of high-In-content InGaN quantum wells because of the lack of strong charge localization and increasingly large internal electrostatic fields. Besides, there are other QCSE- and ultrathin-well-related detrimental features of c-plane InGaN LEDs, including efficiency droop and blue-shift of the emission wavelength (due to carrier screening of internal electrostatic fields) with increasing drive current. Therefore, avoiding QCSE in InGaN LEDs has been considered as an important milestone to realize the ultimate solid-state light sources for general illumination.
In the past few years, tremendous efforts have been made to solve the QCSE obstacle by using the nonpolar (e.g., a- or m-plane) InGaN/GaN structures grown on various substrates. However, the nonpolar approach has its own limitations and challenges such that ideal solutions for the green-yellow-gap and efficiency droop issues are still lacking.